Blue-Green Algae (Cyanobacteria) in Florida waters

Cyanobacteria, also known as blue-green algae, are bacteria that, like plants, use the energy in sunlight to grow (photosynthesis). Cyanobacteria occur naturally in both fresh and marine (salt) water bodies. Most live with other types of algae and microscopic animals in the floating assemblage known as plankton.

Cyanobacteria can multiply quickly and often dominate in lakes with high nutrient levels, particularly when the water is warm and the weather is calm. This proliferation causes blooms that turn the water green, often with floating layers of green scum or mats of green slime that raft up on the downwind shores of rivers and lakes.

Some species of cyanobacteria that live in freshwater do not need much nitrogen in the water because they are able to use nitrogen gas from the air to grow. These nitrogen-fixing algae actually increase nitrogen levels in the water. Lower levels of phosphorus in the water usually limit growth of cyanobacteria and other algae. Therefore, reducing phosphorus levels in freshwater bodies often is the most effective means of preventing algal blooms. However, because some detrimental species of cyanobacteria cannot fix nitrogen, and since nitrogen fixation by cyanobacteria in estuarine and coastal waters is very limited, control of nitrogen pollution still is an important part of a comprehensive strategy to reduce nuisance algal blooms.

Persistent blooms block the sunlight that provides energy to plants rooted in the bottom of lakes and rivers, resulting in the loss of the submersed aquatic vegetation that is valuable habitat. When blooms end, or during prolonged cloudy periods when photosynthesis is reduced, the respiration and decay of the blue-green algae consume oxygen in the water. Low oxygen can affect fish by reducing their ability to forage and grow, flee predation, reproduce, and even survive. The dead algal cells settle to the bottom of water bodies, creating layers of soft sediments that degrade lake bottom habitat.

Blooms of cyanobacteria were recorded as early as 1188. A small number of cyanobacterial species are responsible for most freshwater algal blooms worldwide. Many are from the algal groups (genera), Anabaena, Aphanizomenon, Cylindrospermopsis, Microcystis, and Planktothrix (or Oscillatoria). Several species from these genera occur in freshwater lakes and rivers in Florida.

A species common in Australia, Cylindrospermopsis raciborskii,was first identified in the United States in the 1950s and in Florida in the mid-1990s. Since then it has been identified in many water bodies throughout Florida and the U.S. Cylindrospermopsis raciborskii is a nitrogen-fixing cyanobacterium. It is different from some other bloom-forming algae that occur in Florida because it rarely forms surface scums, and it appears to require less phosphorus to grow.

Cylindrospermopsis was nearly continuously dominant in Lake Griffin in Lake County for at least three years. The St. Johns River Water Management District is working to restore this degraded lake through a multifaceted restoration program, which emphasizes reduction of nutrient levels, particularly phosphorus, in the lake water. Since phosphorus reduction, there has been a reduction in the magnitude and duration of Cylindrospermopsis blooms in Lake Griffin.

Some strains of cyanobacteria produce cyanotoxins. These substances, if ingested in sufficient quantities, can be hepato- or neurotoxic. Cyanobacteria and their associated toxins are believed to have existed in Florida lakes for thousands of years, although not necessarily at bloom concentrations. However, information regarding toxins from cyanobacteria and risks to humans, fish and wildlife is very limited. Also, little is known about the environmental conditions that cause cyanobacteria to begin producing toxins.

Algal toxins are released when the cell dies or is ingested. Most toxic events attributed to ingestion of cyanotoxins involve pets or livestock, when animals drink water from a lake where large amounts of algae have been concentrated along the shoreline due to prevailing winds. Additional information is available in the Fish and Wildlife Research Institute (part of FWC) technical report Resource Guide for Public Health Response to Harmful Algal Blooms in Florida.